This photo of Jupiter and its Great Red Spot was captured by the Hubble Space Telescope on April 21, 2014. Sky-watchers can check out the gas giant for themselves in the early evening sky. Credit: NASA, ESA, and A. Simon (Goddard Space Flight Center)

Even monster planets aren’t immune from downsizing. According to the Hubble Space Telescope, Jupiter’s iconic Great Red Spot appears to have shrunk a couple of sizes.

In this set of images, the diminishing size of the Great Red Spot can be seen from 1995 to 2014, as viewed by the Hubble Space Telescope. Credit: NASA, ESA, and Z. Levay (STScI)

The celestial storm has raged for centuries in the upper atmosphere of Jupiter, long a favorite atmospheric phenomenon for backyard telescope viewing. But while the Red Spot once was large enough to swallow three Earths, new snapshots from the orbiting observatory show that it may now fit only one.

Ground telescope measurements in the 1800s showed the spot spanning some 25,476 miles (41,000 kilometers). By the time the Voyager 2 spacecraft swung by Jupiter in 1979, the storm had shrunk to 14,500 miles (23,335 kilometers) wide. The mysterious shrinking process has apparently continued unabated.

So should we be dropping the superlative “Great” from the name of this monster hurricane?

These changes have been in the making for more than 80 years, so they haven’t completely surprised astronomers. However, it remains a mystery as to why Jupiter’s signature storm is fading. Not only is it at its smallest size ever observed, but it has also changed shape from oval to circular.

One possibility is that some smaller nearby storm cells in the planet’s atmosphere may be draining energy from and weakening the giant hurricane, making it shrink.

“In our new observations it is apparent that very small eddies are feeding into the storm,” said Simon. “We hypothesized that these may be responsible for the accelerated change by altering the internal dynamics of the Great Red Spot.”

Backyard sky-watchers need not fret just yet as the Great Red Spot is still a sight to behold.

To find the largest planet in the solar system with your unaided eyes, just look for a superbright cream-colored star setting low above the western horizon just after nightfall.

Jupiter will sink quickly in the early evenings and will disappear before local midnight.

The fifth planet from the sun is sitting within the boundaries of the zodiac constellation Gemini, just above the right knee of the twin Pollux.

This sky chart shows Jupiter setting in the western sky in the early evening, in the constellation Gemini. Credit: SkySafari

You can’t miss Jupiter—even if you’re stuck within a light-polluted city—as right now it’s one of the most brilliant starlike objects in the entire sky. What makes it such a sparkler? First off, it’s a true monster in size, with a diameter measuring 88,234 miles (142,000 kilometers). More than 1,300 Earth-size worlds could easily fit inside it, making it a wide enough object in the sky to see as a disk even using the smallest of optical aids. Jupiter is also completely shrouded in highly reflective, light-colored hydrogen and helium clouds, which add to its brilliance.

Photo of Jupiter (overexposed) and its four largest moons. This is an approximation of the view through a steadily held pair of binoculars. Credit: A. Fazekas

If you want to have a “wow” moment and really get to know why Jupiter is a favorite with sky-watchers, then try training your binoculars on the planet. Hold them steady enough and you should be able to glimpse four of the planet’s largest moons, first seen by Galileo 400 years ago in 1609. Known as the Galilean moons, they are Io, Europa, Ganymede, and Callisto—all comparable in size to our own moon and named after mythological consorts of Jupiter. You’ll find they look like tiny pinpricks of light beside the bright disk of the planet. (You can get an idea of what you can see with this image that I took from the front steps of my suburban house. It resembles a miniature solar system.)

Your views get even better with a small telescope, which can reveal the planet’s many festoons, knots, tendrils, and turbulent spots in the upper cloud deck of the planet, inside both the polar regions and the brown-colored equatorial belts. At higher magnifications, if you time your observations just right, you will even see hints of the famous Great Red Spot.

Amazing to think that this storm may have been brewing since the time of Galileo in the 1600s and it’s still visible from your own backyard.

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